1. Field of the Invention
This invention relates to a duplexer.
2. Description of the Prior Art
A duplexer serves the purpose of separating the transmitted and received signals in the frequency band of a communication system, such as in a mobile radio telephone standard that uses a frequency division duplex operation or a mixed frequency division/time division duplex operation. The frequency band of the communication system is divided into a transmission band and a reception band. A duplexer should ensure a lower insertion attenuation in the relevant passband, as well as a high insulation of the reception and transmission paths, i.e., a high suppression of a signal to be transmitted in one direction in the passband of the signal to be transmitted in the opposite direction.
FIG. 1a schematically shows a known duplexer that has an antenna port ANT, a transmission input TX-IN and a reception output RX-OUT. The transmission input TX-IN can be connected to a power amplifier PA and the reception output RX-OUT can be connected to a low noise amplifier LNA. A transmission TX is arranged between the antenna port ANT and the transmission input TX-IN. A bandpass filter is arranged in each signal path (reception bandpass filter RX-BP in the reception path RX and a transmission bandpass filter TX-BP in the transmission path TX). All ports of the duplexer, i.e., ANT, TX-IN, and RX-OUT, are designed asymmetrically.
In addition, the publication United States Publication No. 2003/0060170 discloses a duplexer schematically illustrated in FIG. 1b, in which all ports are designed symmetrically.
It is known to arrange an adapter network between the antenna port and the bandpass filter TX-BP and/or RX-BP. In addition, an adapter network or an impedance transformer can be used at an intermediate stage to adapt the impedance of the transmission input TX-IN and the power amplifier and/or to adapt the impedance of the reception output RX-OUT and the low noise amplifier.
As used herein, “impedance transformer,” means a circuit arrangement that performs a transformation (matching) of the output impedance of the first stage into the input impedance of a post-connected second stage, wherein these impedances differ from each other significantly, by at least a factor of 2. A smaller impedance adaptation between the stages of a circuit arrangement can be performed using an adapter network.
Another known design is the use of a balun between a reception bandpass filter and the low-noise amplifier to balance the reception signal. By contrast, the output of a performance amplifier is usually designed asymmetrically.
The circuit elements that are pre-connected and post-connected in relation to the duplexer form an interface to the duplexer, at which point, in the case of a sudden impedance jump, a portion of the energy of the signal is reflected, which is why an additional impedance adaptation of the stages is often required, and which results in signal losses at the adaptation elements.